it will make a difference across the the rpm range when you put your foot into it, something reasonable to expect would be about 10hp 10ft of torque more I would do headers and the cam too just to make the package. The 68 and up 302 4v type heads are kind of rare and only really just have a smaller chamber that bumps up compression. The two barrel heads are fine you have them I would stick with them and just do the cam at the same time since you will have the intake off you might as well.